reversers vs brakes

[avionneta]

hello,
I'm looking for info about the cost of using reversers on engine maintenance versus the cost of the brakes wear when you don't use them ?

[Sean Dell]

I think if you spoke to Mr Airbus - he would be happy just with brakes. Indeed I am led to believe that the A380 was originally designed with no reversers - it was just the FAA that insisted on inboard (?) reversers being present to allow for certification.

HTH

[Canuckbirdstrike]

Simple question, complex answer. The cost issue is dependent on whether you are using steel or carbon brakes and how your brake and engine maintenance costs are attributed (part of lease cost, per cycle, hourly etc.)

Overall from a cost perspective with carbon brakes it is more cost efective to use more brakes and less reverse. The decision must be adjusted for each landing to cater to weight, landing distance available, runway condition and turnaround time.

[varigflier]

Doesn't the FARs say to use the reversers whenever possible as soon as touchdown to decrease the landing distance? I see people using just brakes all of the time....

[lomapaseo]

If you factor in the cost of ownership, including maintenance, it's far cheaper to not even own a reverser and its maintenance. Of course then you are going to have to rely on both the pilots and the brakes not to overextend the safety margins presumed in each legal landing.

I like an occasional backup like a chute, or an arrestor system and forget the reversers

[yrvld]

sorry, not trying to be sarcastic, but are you pilots or economists?

[Canuckbirdstrike]

Small clarification on my earlier post. I do not advocate or support using NO reverse. The best option is to use IDLE reverse when operationally safe and feasible. This is the best balance of cost vs. options for safety. This allows the selection of more reverse if required for unforeseen circumstances without having to wait for them to deply.

[john_tullamarine]

Oh dear ... gone are the days when one would rip in a fistfull of reverse on touchdown and savour the engines' sweet song on the landing roll ...

MFS has queried a comment I made here (and now have deleted) regarding certification aspects of reverse thrust .. as I can't lay my hand on the regulatory derivation, it is best that the comment be removed to avoid confusion. It may be that my memory has placed the requirement in the wrong jurisdiction ...

[G-ALAN]

On a wet or standing water/contaminated runway wouldn't reverse reduce the landing roll considerably?

[firebird_uk]

Here in my ATPL Aircraft General Knowledge manual it says "On a jet most of the retardation on the landing or rejected take-off comes from the brakes rather than the thrust reversers."

And if that's what the JAA say I guess it must be true. :}

But what do I know, I'm taking an ATPL(H) and having to study 737 landing gear systems! You know it makes sense. :ugh:

[Brian Abraham]

but are you pilots or economists?

Pilots, pilots?????? No such thing these days, systems managers please!!!! Do keep up. :rolleyes:

[ERJFO]

Doesn't the FARs say to use the reversers whenever possible as soon as touchdown to decrease the landing distance? I see people using just brakes all of the time....

No. For an FAA governed airline to even fly to an airport the airplane must be able to stop on 60% of the available runway with breaks and spoilers only.

Most airlines now are changing their SOP from using full reversers on landing to using idle reversers (deployed but not spooled) and breaking unless more stopping power is needed at the descretion of the crew.

[tom775257]

It is interesting, my airline asks us to use max reverse every landing when no noise abatement is in force, and use minimum brakes. At our base airfield which has a very long runway, means I rarely ever touch the brakes above 70kts or so when I bring the reverse back to idle.

I personally don't worry about the economics of brakes vs. reverse, I do what the company want when performance and airmanship permits. Our aircraft are leased, so perhaps this means engine wear isn't considered as much? Who knows, I like it, I get to make lots of noise ;)

We have very new 320s, carbon brakes.

Tom.

[Penworth]

Conversely, my airline (737/757) tells us to use idle reverse only with most of the stopping coming from the brakes. This is undoubtedly what they've worked out is the most economical technique for our aircraft. And as canuckbirdstrike says, this has the benefit of allowing more reverse thrust if required. I may be wrong but I seem to remember reading that brake life also increases with a steady higher autobrake selection (2 or 3) as opposed to minimum autobrake. This would be another argument for the way its done in my airline.

PW

[GlueBall]

QF1, B747-438 overrun at BKK 23 Sep 1999 demonstrated that QF's "no reverse" procedure was flawed. The accident report also had stated the manufacturer's recommendation to use reverse thrust for all landings.

[p1fel]

I remember reading a detailed study carried out by Lufthansa for their fleet of Airbus and Boeing aircraft, re costs for brake wear/replacement, fuel costs, engine wear, blocker door wear, etc.
The conclusion was for all fleets use 70% N1 Reverse and dependant on fleet, either Medium or Auto Brake 2 setting.
I caution this may well be out of date at todays costings as this study was carried out in the early 90's.

[BelArgUSA]

I personally require (classroom, simulator or line) the deployment of reversers (even if only at idle) to cancel the "residual thrust" of engines. On the 747s, we use minimum brakes on dry runways, and medium brakes on wet runways. Landing without reversers deployed is against our SOPs. They stay deployed (at idle) until about to start the turnoff. We also recommend to use full length of the concrete... no hurry to get these "high speed" turnoffs, we much prefer to add 1/10th of flight pay for taxi-in time.
xxx
Old military habit... I still do aerodynamic braking as well. My privilege, thanks to my position and seniority... I hear some of you objecting "you will hit the tail, one day"... Well, in about 22,500 hrs of flying, including stretched DC-8s, I never scraped the tail, and with only a few months left in my career, unlikely I will do... knock on wood.
xxx
In 90% of my landings (all are on 10,000+ feet long runways), I guarantee that I turn off at the end of the runway without use of brakes, as I disconnect autobrakes, after satisfactory touchdown on dry runways. Brakes remain cold, put your hands on them, if you do not believe me.
xxx
And as far as "medium breaks" are concerned, they are 15 minutes long for classroom/simulator sessions.
xxx
:)
Happy contrails

[pakeha-boy]

Quote Brian.....Pilots, pilots?????? No such thing these days, systems managers please!!!! Do keep up.

well said mate and absolutely true......flew the F-28 for several years,no reverse,just brakes.....worked just fine.....fly the 321 now and have reversers,and poor brakes.....works just fine....

.reallly dont care about the cost....you need them to take-off and land....their costs?????? ...it called the costs of doing buisness!!!

[avionneta]

what is generally accepted for 747 is not for medium weight aircraft, I mean, to let go the aircraft to the end or the first after the speed is very slow. you exit where you want anyway but minimum runway occupancy time must probably wait for medium jets to use there brake

[BelArgUSA]

¡Basta, Che!
xxx
I am certain you love to follow the 747s "very close"... dont you...?
Try it 30 seconds behind... and remember, at LEMD, I do "1 dot" above glide slope.
Feliz acercamiento para vós...
xxx
:}
Happy contrails

[rubik101]

To those of you asking about stopping distances with regard to use of reversers; remember that the auto-brake selects a deceleration RATE, hence the total stopping distance will be the same with or without the use of reverser. Applying the standard 70% reverse will simply reduce the amount of braking applied, hence reducing wear and brake temps.

[john_tullamarine]

the airplane must be able to stop on 60% of the available runway

.. but be aware that, on a limiting runway, you had best push the pedals through the floor to achieve something approaching the raw data figures ...

Most who have had any exposure to performance landing tests have a jaundiced view of such comments ..

I'd really hate to get near the far end of the seal and then find out I have a problem .. as opposed to putting some effort into slowing down early and then finding out that I don't have a problem ... but call me a dinosaur, I guess.

Company economics are important .. but will not cut much slack for you at the enquiry after you bend the bird ... as a very wise checkie observed at the debrief years ago .. the book usually has the most important words left off the front page .. 'to be read with a bit of commonsense'

[BelArgUSA]

Fully agree with you, John T...!
xxx
To the few of you flying some airplanes without reversers, such as Learjets, with drag chutes... just a word...
If you think you need to use the drag chute, do it right after touch-down...
They work great... except with bad crosswinds.
xxx
Obviously, you do not want to have to go out of the airplane to pack the drag chute after use...
What a mess of dirt, mud, and... all that on your new uniform...
Then you trip face down with your legs tangled in the ropes...
As a general rule, the junior pilot is on duty to pick-up the fabric...
xxx
I wrote "deploy immediately after touch down"... but you did not read that...
So, you wait until 50-60 knots to decide, and the end of the runway is coming...
Too late...! At that speed, the drag chute has same effectiveness as a piece of wet Kleenex tissue.
But you will keep the uniform clean... Just dirty shoes with the mud beyond the end of the stopway.
xxx
:*
Happy contrails

[AirRabbit]

The regulations (in the US, anyway) don’t require the use of reverse thrust to calculate landing distances or accelerate-stop distances, but the rules don’t prohibit that use either. What they do say, in both situations, is that another “means” may be used other than wheel brakes provided whatever is used is safe and reliable; is used so that consistent results can be expected in service; and is such that exceptional piloting skill is not required to control the airplane.

In fact, you might recall the Southwest accident at Midway airport some time back? At that time SWA used an “on-board” calculator (some said it was a laptop computer that the F/O lugged around) to calculate the landing distance required using reverse thrust – meaning that it had to work. I’m under the impression that they don’t use that same procedure anymore.

[PantLoad]

Of course, your company's SOP is what matters...nothing else.

At my company, the 'normal' landing uses max reverse thrust...sans local noise requirements and any other special airport requirements, MEL, non-normals, etc.. However, it IS in our SOP that the captain can elect to use idle reverse, if he feels conditions are such that it is prudent to do so.

Interestingly, we are required to use autobrake on every landing (except for MEL and other situations that require otherwise). The logic is that, with carbon brakes, the number of brake applications, primarily, determines brake wear. And, by requiring autobrakes on every landing, ostensibly only one brake application is used for each landing.

Personally, I use max reverse on every landing, except where prohibited (as stated above). However, I've noticed...and this is just a non-scientific observation...that the leading edges of the engine fan blades seem to get less chewed up when guys use idle reverse. (I may be totally full of XXXX on this one...so, I qualify this...feel free to take pot shots at me!:p) Most of us try to do rolling takeoffs for this reason, as well.

Fly safe,


PantLoad

[Brian Abraham]

MFS has queried a comment I made here (and now have deleted) regarding certification aspects of reverse thrust .. as I can't lay my hand on the regulatory derivation, it is best that the comment be removed to avoid confusion. It may be that my memory has placed the requirement in the wrong jurisdiction ...

Didn’t dig out any regs John but from the BKK overrun report

Certified landing distance = from 50 feet above the landing surface in the landing configuration at an airspeed of 1.3Vslg to where the aircraft comes to a stop using maximum wheel braking
Landing distance (Dry) = 1.67*certified landing distance
Landing distance (Wet) = 1.15*dry landing distance
Reverse thrust is not used in these calculations because it is considered as an additional safety factor.

Letter from Boeing to QANTAS July 1997

…..Additionally, some of the techniques we have heard discussed, such as reduced landing flap settings and the use of idle reverse thrust, have a negative impact on airplane stopping performance. Therefore, these techniques are not recommended as standard practice.

Extract from report into QANTAS overrun Bangkok 23 September 1999.

The flight crew did not select (or notice the absence of) full reverse thrust. The use of full reverse thrust would have substantially reduced the landing distance on a runway with poor braking action. The failure of the crew to consider the use of full reverse thrust during the landing roll appeared to be primarily due to the high workload they were experiencing. Had the crew received more training in the importance of reverse thrust on water affected runways, or recent experience in the use of reverse thrust, it is reasonable to expect that the crews awareness of the importance of reverse thrust (and therefore the likelihood of them selecting full reverse thrust) would have been greater.

QANTAS at the time had no simulator exercise requiring the use of full reverse thrust. Idle reverse was the ‘standard’ procedure.

Letter from Boeing to QANTAS April 2000

Boeing does not consider the standard practice of going to reverse idle (idle detent) only to be patently unsafe, but does think that it reduces the existing performance margins. It is acceptable pilot technique to do this (using good judgement) as an exception to the normal procedures when landing on a long, dry runway. We perceive, however, that there is a human factors issue of developing a habit pattern of not using reverse thrust beyond the idle detent. The pilot may then fail to respond quickly when such reverse thrust is needed during an RTO (rejected takeoff) or landing in some type of performance critical situation. We therefore do not provide a “No Technical Objection” for this as a standard operating policy.

If the reverse idle technique is adopted, it should be taught as the exception rather than the rule. Further, we would encourage simulator drills to be incorporated into the transition and recurrent training courses that would require pilots to use judgement to use full reverse thrust as the best successful means of stopping the airplane. This would periodically reinforce this concept of using that capability when needed.

Do the Boeing manuals not advocate the use of full reverse thrust as the BKK report suggests?

[Centaurus]

The best option is to use IDLE reverse when operationally safe and feasible. This is the best balance of cost vs. options for safety. This allows the selection of more reverse if required for unforeseen circumstances without having to wait for them to deply

That is a fallacy and a potentially dangerous one at that. If you use idle reverse after touch-down and then as the aircraft slows down you realise that due to high groundspeed towards the end of the landing run you need reverse in a hurry - then it is too late because by then the reverse thrust efficiency even with full reverse applied is degraded. From 100 knots down, reverse thrust is less effective and markedly so below around 80 knots.

Idle reverse is probably better in the high speed regime than no reverse but not by much. You can count on hot brakes when using just idle reverse. The Boeing 737 FCTM states that the use of minimum reverse thrust as compared to maximum reverse thrust can double the brake energy requirements and result in brake temperatures much higher than normal.

[BelArgUSA]

OK... I will quote my "bible" here, FCTM 747-200, for JT9D-7Q engines.
At the end, I will pass along some personal techniques, and suggestions as I am active as check-captain.
xxx
(FCTM) Reverse Thrust
To obtain maximum reverse thrust, after the interlock release, pull the reverse levers up to the stops. Maintain maximum reverse thrust until the airspeed approaches 60 knots. Start modulating the reverse thrust approaching this speed so that the reverse levers are moving down at a rate commensurate with the deceleration rate of the airplane. A reverse thrust level of 60% N1 should be reached by 40 knots. This reverse thrust level may be maintained to a safe taxi speed or a full stop if required. It is important that the reverse thrust levers ve modulated towards the lower thrust settings at the designated airspeed to prevent engine surging.
(...)
Changes of command to the trust reversers while in the mid-stroke between reverse and forward thrust or vice-versa can damage the actuating mechanisms. They require approximately 2 seconds to extend, and 5 seconds to stow to forward thrust.
(...)
The use of reverse thrust below approximately 40 knots should be avoided (unless required for adverse conditions) due to possible surging and foreign object ingestion. - CAUTION - Do not attempt a go-around after reverse thrust has been initiated. Factors dictating this are - 5 seconds are required for a reverser to stow in the forward thrust position, and a possibility exists that a reverser may not stow in the forward thrust position.
(...)
FCTM - Auto Brakes
MAX - This setting should only be used when a minimum stop distance is mandatory. The distance to a full stop will be comparable to the actual certified landing distance on a dry runway. Operation of this setting results in a full 3000 psi being applied to the anti-skid modules. Reverse thrust does not result modulation of auto brake pressure but is additive to the total deceleration level achieved.
MED - This setting should be used for wet or slippery runways or when positive braking is imperative, and for all landings where rollout distance is limited. Operating at this setting limits applied brake pressure to 1800 psi, and will provide a wet or slippery rinway deceleration with optimum anti-skid performance. Thrust reverser operation will result in modulation of the applied brake pressure.
MIN - This setting will provide a moderate deceleration effect suitable for all routine operations. Thrust reverser operation will result in modulation of the applied brake pressure.
(...)
My notes in briefings...
We generally fly approaches at Vref+5 Kts, with 25 flaps for reduction of noise and fuel burn. I stress the use of 30 flaps when runways are wet/slippery for touch down. When landing is assured, at 50 feet over the threshold, it is acceptable to select flaps 30 if approach was performed with 25 flaps. Let the speed bleed off, with no power increase. Further speed reduction from Vref+5 to Vref when 30 feet above runway (start of power reduction and flare) is acceptable, probably achieving a Vref-5 touchdown.
xxx
I personally suggest immediate pull of the 4 reversers upon touch down, which is indicated by deployment of speed brakes. I suggest immediate full operation of inboard reversers (little concern for asymetric power) then careful operation to full reverse of the outboard engines, in case of power asymetry. In case of engine out (such as in training) for an outboard engine, werecommend deployment of opposite engine to idle reverse only. For the case of inboard engine, we recommend partial reverse thrust.
xxx
Minimum speeds for use of reverse, as copied above from the FCTM. My experience has shown that these are "inboard engines" limitations, as the reverse flow of inboards disrupts the inlet of outboard engines, which are located further aft of the inboards. When moduling power to reduce the reverse thrust, I fully abide by the reduction, however I "deduct" 10 knots for the outboard engines... in practice, reducing the inboards as published to achieve 60% N1 by 40 knots for the inboards, but leaving more power ("1 extra knob worth") on the outboards, to achieve 60% N1 by... 30 knots.
xxx
A few years ago, on a slippery short runway in Chicago ORD, one of our 747 ended with nosewheel beyond the end of the runway, because the PF slammed the reversers from high power setting reverser (to a full stop) but got a burst of unexpected foward thrust, after the reversers did stow. So, my personal technique, is to stay with idle reverse, until airplane taxiing speed (and turnoff, if applicable) is achieved.
xxx
:)
Happy contrails

[john_tullamarine]

Judd,

Your observation probably wasn't terribly helpful, either in its own right or in thread context ... as indicated by the subsequent chastising responses.

Perhaps we might ask that you consider the content of your posts with a little more contemplation than seen in your last ?

[palgia]

CENTAURUS

From 100 knots down, reverse thrust is less effective and markedly so below around 80 knots.


Would you be so kind to explain why the amount of reverse thrust (ie. force) markedly decreases with decreasing speed? and why "markedly so below 80kts"????:confused::confused::confused:

Me thinks the reverse force remains roughly equal during deceleration. Of course, the impact on landing distance is hugely dependent on what speed you initiate reverse.... but thats a different story and it applies to any type of retarding force.

[WHBM]

Contrary to comments about not using reversers at lower speeds, US operators use reverse thrust in some cases to back the aircraft away from the gate without external assistance, in a procedure known as "powerback".

In Europe this seems unknown, and when you first see it done in the US you can't quite believe it. The power level seems very high for the slow rearward speed attained - slower than a tug might push at. Why is this acceptable in the US but not elsewhere ?

[skiesfull]

Noise - many major airports outside the USA require idle power only on the ramp. There is also the question of FOD.

[Centaurus]

Would you be so kind to explain why the amount of reverse thrust (ie. force) markedly decreases with decreasing speed? and why "markedly so below 80kts"????
Doubtless there is a perfectly logical mathematical answer. But in simple pilot language reverse is more effective at high airspeed because the forward direction vector of the reverse thrust is pushing hard up against the dynamic force of the airstream over the aircraft. Equal and opposite reaction and all that jazz. As the aircraft slows the opposing force to the reverse thrust forward vector is less and the deceleration is less. No doubt it all sounds a bit iffy to the educated but it's pretty close to the truth. If you fly a glass cockpit aircraft you can see the speed trend green arrow indicating quite high deceleration rate as reverse cuts in shortly after touch down (discounting spoiler drag and brakes of course) Then around 100 knots (depending on the type of aircraft) the arrow starts to steadily reduce in length as the aircraft slows even with full reverse applied. Again this may be an over-simplification but it is basically true.

[Mad (Flt) Scientist]

Because the amount of reverse thrust consists of two components: the ram drag (which as usual is a function of the amount of air going in the front end, and its speed) and the gross thrust (which, unusually, is being directly 'vaguely' forwards, with various engines doing so to varying degrees of effectiveness).

The gross thrust term is more-or-less independent of the forward speed, and depends on basic engine parameters. Important here are the amount by which the flow is directed forward - often not much, indeed some designs don't divert the core flow at all. This is one reason why powerback is often a problem (the other being reingestion).

The ram drag term is basically the mass flow into the intake times the forward velocity. Since the mass flow is also dependent on the velocity, this makes it a velocity^2 term. So the ram drag term at 100knots is (10/8)^2= 56% higher than at 80knots, and (10/6)^2=178% higher than at 60knots. Or, if you prefer, if 100knots=100% reverse thrust, then I only have 64% at 80knots and 36% at 60knots.

Since the ram drag term can be a significant proportion of the total reverse thrust, I can easily have lost 50% of my reverse thrust by 60knots or so. That means a potentially significant drop in deceleration.

Next, what really matters in terms of stopping isn't deceleration - that's just a means to the end, which is distance. From the simple equations of motion, distance covered, s, between speeds u and v at a constant acceleration is:

s= (v^2 - u^2) /2a.

Between, say, 110 and 90 knots, the bit in the brackets is -4000.
Between, say, 80 and 60, it's -2800.
So if I could gain, or lose, 10% of my decel, it would be worth 40/28 times more in terms of distance if I could arrange to get the extra decel over the higher speed range. Basically, it's like compound interest; every little counts at the start of the decel run, but by the time you're 'trundling', the distance isn't so sensitive to the exact accel/decel.

So, two things at work:
1. Due to the way most reverse thrust is actually implemented, you get more actual decel at higher speeds; and
2. Any decel at high speed is much more effective in reducing landing roll than the same decel at lower speeds.

[lomapaseo]

Reverse thrust to back out of the gate

... There is also the question of FOD.

Question answered. you end of backing away from the reverse efflux not running into it. So no increased FOD problem unless you power up enough to suck a vortex off the ramp before you move